Phytoremediation of 137Cs: factors and consequences in the environment

A critical review about phytoremediation of heavy metals and radionuclides: from mechanisms to post-remediation strategies

Phytoremediation has emerged as an environmentally friendly and cost-effective solution for mitigating heavy metal and radionuclide contamination in soil and water. While extensive research has been conducted on phytoremediation mechanisms and the effectiveness of various plant species in pollutant uptake, limited attention has been given to the crucial aspect of post-remediation biomass management, particularly for biomass containing heavy metals and radionuclides. This review provides a pioneering perspective by integrating phytoremediation mechanisms with a comprehensive discussion of post-remediation biomass treatment methods, such as incineration, solidification, gasification, and pyrolysis, which are essential for reducing environmental risks. This study's output highlights that solidification is more suitable for radioactive biomass management for safe long-term storage and sustainable radioactive waste management; however, it does not produce value-added products. Meanwhile, gasification offers relatively low-emission biomass treatment compared to incineration and enables superior energy conversion efficiency and lower costs on a large scale compared to pyrolysis. The findings contribute to improving the overall efficiency of phytoremediation and provide insights into post-remediation biomass handling methods, reinforcing the feasibility of phytoremediation as a sustainable large-scale remediation solution. By identifying research gaps and proposing future directions to enhance the sustainability of phytoremediation, this review serves as an advantageous reference for policymakers, researchers, and environmental practitioners in designing effective phytoremediation strategies and post-remediation biomass management policies.

Assessment of radioisotope concentrations of 137Cs and 90Sr in the herbaceous phytocenoses plants of the Dnieper river floodplain ecosystems (northern Ukraine)

The experimental data on the 137Cs and 90Sr concentation in the herbaceous phytocenoses of the Dnieper floodplain ecosystems in northern Ukraine have been discussed. Different radionuclide contents in plants and soils of different ecosystems of the floodplain depends on the herbaceous phytocenoses proximity to the river bed: a higher content of radiocaesium has been found in plants and soils of the meadow and marsh ecosystems, which are close to the river bed, and radiostrontium in the marsh and psamophyte ecosystems. Besides the intensity of radionuclide uptake by plants has been determined as by the soil and the cenotic habitat conditions so the biological characteristics of the species. The species specificity of the 137Cs and 90Sr accumulation by plants and the direct dependence of the soil-to-plant transfer factor from the density of the floodplain ecosystem contamination with radionuclides have been proved. Our results testified that significant differences in the caesium and strontium distribution were observed between ecosystems, likely influenced by soil characteristics, vegetation types and biomorphs. Our data suggest that the ecosystem type played a significant role, with caesium being most abundant in ruderal ecosystems and least abundant in psamophytic ecosystems, while strontium effects were particularly high in ruderal and psamophytic ecosystems compared to meadows.

Advancements in microbial-mediated radioactive waste bioremediation: A review

The global production of radioactive wastes is expected to increase in the coming years as more countries have resorted to adopting nuclear power to decrease their reliance on fossil-fuel-generated energy. Discoveries of remediation methods that can remove radionuclides from radioactive wastes, including those discharged to the environment, are therefore vital to reduce risks-upon-exposure radionuclides posed to humans and wildlife. Among various remediation approaches available, microbe-mediated radionuclide remediation have limited reviews regarding their advances. This review provides an overview of the sources and existing classification of radioactive wastes, followed by a brief introduction to existing radionuclide remediation (physical, chemical, and electrochemical) approaches. Microbe-mediated radionuclide remediation (bacterial, myco-, and phycoremediation) is then extensively discussed. Bacterial remediation involves biological processes like bioreduction, biosorption, and bioprecipitation. Bioreduction involves the reduction of water-soluble, mobile radionuclides to water-insoluble, immobile lower oxidation states by ferric iron-reducing, sulfate-reducing, and certain extremophilic bacteria, and in situ remediation has become possible by adding electron donors to contaminated waters to enrich indigenous iron- and sulfate-reducing bacteria populations. In biosorption, radionuclides are associated with functional groups on the microbial cell surface, followed by getting reduced to immobilized forms or precipitated intracellularly or extracellularly. Myco- and phycoremediation often involve processes like biosorption and bioaccumulation, where the former is influenced by pH and cell concentration. A Strengths, Weaknesses, Opportunities, and Threats (SWOT) analysis on microbial remediation is also performed. It is suggested that two research directions: genetic engineering of radiation-resistant microorganisms and co-application of microbe-mediated remediation with other remediation methods could potentially result in the discovery of in situ or ex situ microbe-involving radioactive waste remediation applications with high practicability. Finally, a comparison between the strengths and weaknesses of each approach is provided.

The surface of small glaciers as radioactive hotspots: Concentration of radioisotopes during predicted intensive melting in the Alps

Glaciers are considered secondary sources of pollutants, including radioisotopes such as Cesium or Plutonium, with heightened concentrations compared to other ecosystems. The predicted melting of glaciers poses a substantial risk of releasing stored radioisotopes, yet understanding the glacier-specific factors influencing their concentration remains limited. This study investigates the relationship between glacier altitude, surface area, organic matter content in dark supraglacial sediment (cryoconite), precipitation, and activity concentrations of natural (210Pb) and anthropogenic radionuclides (137Cs and 241Am) across 19 Alpine glaciers. Results indicate that radioisotope concentrations depend on organic matter content in the cryoconite, highlighting the role of biotic-abiotic interactions in pollutant accumulation on glaciers. Moreover, 210Pb activity concentration decreases with glacier altitude, likely due to atmospheric variations in 222Rn. Water precipitation events, such as during peaks in 137Cs deposition and after the Chernobyl Nuclear Power Plant disaster, do not impact current activity concentrations. Importantly, radioisotope concentrations in cryoconite are higher on smaller glaciers. This directly supports the hypothesis that the cryoconite retains a significant share of radioisotopes stored in the ice during intensive melting. Since many small glaciers in the Alps are predicted to disappear within the next 50 years, we anticipate release of radioisotopes to mountain ecosystems might be higher than previously forecasted.

Role of independent research at AERB for ensuring safety of nuclear facilities in India

The mission of Atomic Energy Regulatory Board (AERB) of India is to ensure that the use of ionising radiation and nuclear energy in India does not cause unacceptable impact on the workers, members of the public and to the environment. AERB has the mandate to carry out detailed safety review for the siting, construction, commissioning, operation and decommissioning of nuclear and radiation facilities established within the country. To deliver and maintain a strong, credible and technically sound regulation, AERB has established the Safety Research Institute (SRI) at Kalpakkam with a robust technical infrastructure and wide knowledge base. This paper highlights the independent safety research activities carried out at SRI and its role to support and facilitate the decision-making process by AERB at various stages of regulatory review for ensuring safety of the nuclear facilities in India.

Impacts of soil type and drought stress on growth and cesium accumulation in Napier grass

In our previous study, the decontamination efficiency of cesium-137 (137Cs) by Napier grass (Pennisetum purpureum Schum.) in the field was shown to be variable and often influenced by natural environmental factors. To elucidate the factors influencing this variable 137Cs-decontamination efficiency, we investigated the influences of soil type and drought stress on Cs accumulation using cesium-133 (133Cs) in Napier grass grown in plastic containers. The experiment was performed using two soil types (Soil A and B) and three different soil moisture conditions: well-watered control (CL), slight drought stress (SD), and moderate drought stress (MD). Overall, our results indicate that soil type and drought have a significant impact on plant growth and 133Cs accumulation in Napier grass. Plant height (PH), tiller number (TN), leaf width (Wleaf), and dry matter weight of aboveground parts (DWabove) and root parts (DWroot) in Soil B were greater than those in Soil A. Drought stress negatively affected chlorophyll fluorescence parameters (maximal quantum efficiency of photosystem (PS) II photochemistry and potential activity of PS II), PH, TN, Wleaf, DWabove, DWroot, and total 133Cs content (TCs), but it had a positive effect on 133Cs concentration. The 133Cs concentration in the aboveground parts (Csabove) was increased by MD approximately 1.62-fold in Soil A and 1.11-fold in Soil B compared to each CL counterpart. The TCs in the aboveground parts (TCsabove) decreased due to drought by approximately 19.9%-39.0% in Soil A and 49.9%-62.7% in Soil B; however, there was no significant effect on TCsabove due to soil type. The results of this study indicate that soil moisture is a key factor in maintaining Napier grass 137Cs-decontamination efficiency.

Phyto- and bio-management of metal(loid)-contaminated soil by inoculating resistant bacteria: evaluating tolerance of treated rice plant and soil with its efficiency

Anthropogenic activities are increasing the amount of heavy metals and metalloids in the environment on a global scale, harming all living things and necessitating the employment of bioremediation procedures. Metal-resistant bacteria were used to clean polluted soil and promote plant growth; this approach has gained attention in recent years for bioremediation of heavy metal-contaminated systems. We studied the effects of chromium and lithium in Oryza sativa under controlled conditions. In the present study, lithium concentration was applied 50 ppm to 200 ppm according to the dose tolerance level, while the concentration of chromium was 10 ppm throughout the experimental setup due to its concentration observed up to 10 ppm in the targeted soil, which is present in Kasur area Punjab, Pakistan, for rice crop production in future perspective. The results reflect that plants with high lithium concentration have shown decreased plant growth and development, but due to bacterial presence, they thrived until harvesting stage. Due to increase in stress concentration up to 200 ppm, decline in plant growth was observed, but after bacterial inoculation, better growth was seen (chlorophyll content increased to 40, and panicle numbers were more than 13). Our findings reveal that lithium and chromium have a direct negative impact on Oryza sativa, which can be minimized by utilizing halophilic microbes (Klebsiella pneumonia and Enterobacter cloacae) through soil-plant system.

A magnesium transporter is involved in the cesium ion resistance of the high-concentration cesium ion-resistant bacterium Microbacterium sp. TS-1

Cesium ion (Cs⁺) resistance has been reported in bacteria but is poorly understood as reports on Cs⁺-resistant bacteria have been limited. We previously reported a novel Cs⁺/H⁺ antiporter CshA implicated in Cs⁺-resistance in Microbacterium sp. TS-1. The present study used the same screening method to isolate novel Cs⁺-sensitive mutants and their revertants from TS-1. A comparative mutation site analysis using whole-genome sequencing revealed that MTS1_03028 encodes the Mg²⁺ transporter MgtE and is a candidate Cs⁺ resistance-related gene. We performed a bioinformatic analysis of MTS1_03028 and complementation experiments on Cs⁺ resistance in the TS-1 MTS1_03028 mutants Mut5 and Mut7 as well as Escherichia coli expressing MTS1_03028 in the presence of Mg²⁺. We established the role of MgtE in Cs⁺ resistance through a functional analysis of TS-1. Enhancing Mg²⁺ transport by expression of MTS_03028 conferred increased Cs⁺ resistance. When this strain was exposed to Cs⁺ concentrations exceeding 200 mM, CshA consistently lowered the intracellular Cs⁺ concentration. To our knowledge, the present study is the first to clarify the mechanism of Cs⁺ resistance in certain bacteria. The study findings offer important insights into the mechanism of bacterial resistance to excess Cs⁺ in the environment, suggesting the potential for bioremediation in high Cs-contaminated areas.